"What we found was that 104 out of the 109 lice populations we tested had high levels of gene mutations,” said Kyong Yoon, who conducted the research. “Which have been linked to resistance to pyrethroids."

Pyrethroids are a type of incesticide that include a chemical also used in many lice treatments widely recommended by doctors and schools and available in most drug stores.

Yoon gathered lice from 30 states for the study. He found that the lice had a trio of genetic mutations known as “knock-down resistance.” The mutations affect the insect’s nervous system, desensitizing it to pyrethroids, according to the study.

Yoon found the three mutations in lice in 25 states. New York, New Jersey, New Mexico and Oregon had one, two or three mutations. Michigan was the only state tested that is still susceptible to the mutation.

"If you use a chemical over and over, these little creatures will eventually develop resistance," Yoon said. "So we have to think before we use a treatment. The good news is head lice don't carry disease. They're more a nuisance than anything else."